Safety risk definitions for results alignment

The number of injury crashes that have occurred in the last 5- or 10-year interval is determined using relevant data from the Crash Analysis System (CAS).

By using all injury crashes instead of just fatal and serious crashes to predict the number of DSI likely to occur in future, the number of crashes analysed increases, and the impact of individual high severity or low severity crashes are normalised.

For assessments completed after 31 March each year, include crash data from the previous calendar year in the analysis. See 'Calculating DSI casualty equivalents' for approved methods for calculating estimated DSI casualty equivalents.

Note:

• Crashes can take up to three months to appear in CAS, therefore if you complete your assessments after 31 March each year, include the previous calendar year of information in the five-year analysis period.
• If you prepare your assessments earlier in the calendar year you may use an older five-year analysis period.

If a corridor or intersection has a high or medium-to-high collective safety risk, it becomes a high overall safety risk for the purposes of results alignment.

If a corridor or intersection has a medium or low-to-medium collective safety risk, you can calculate personal safety risk using either injury crash data or infrastructure risk rating(external link):

• The personal safety risk calculation requires information on corridor or intersection traffic volumes. If these calculations determine that the corridor or intersection has a high or medium-to-high personal safety risk, then it becomes a high overall safety risk for the purposes of results alignment.
• The infrastructure risk rating (IRR) is shown on the Waka Kotahi Safer Journeys Risk Assessment Tool(external link). To calculate IRR, you need eight additional variables. The information required to populate these eight variables is available through Google Street View and aerial imagery or can be manually calculated from site inspections and measurements. Once the IRR is determined, you can calculate a high, medium or low overall safety risk (as set out below).

Please use the information below to calculate safety risk and to determine whether the overall safety risk is high, medium or low for the purposes of results alignment.

To achieve a very high results alignment, the activities or interventions should:

• implement a speed management approach focusing on treating the top 10 percent of the network that will result in the greatest reduction in deaths and serious injuries
• target areas of high collective risk with high DSI reduction measures that achieve a DSI reduction of at least 40%. Treatments that achieve this are typically, but not limited to, primary Safe System interventions as outlined in Austroads documents such the Safe System Assessment Framework(external link).

   

You can assess a corridor as a high overall safety risk using one or more of the following methods.

High overall safety risk for corridors is where:

1. The estimated DSI casualty equivalents, based on the latest five-year period, result in a high or medium-to–high collective risk. This is determined using information from figure 1 below for urban (speed limit ≤70km/h) or figure 2 below for rural (speed limit ≥80km/h) speed environments respectively, and have a minimum of three injury crashes in five years.
2. The estimated DSI casualty equivalents per 100 million vehicle kilometres travelled (based on the latest five-year period) are ≥12 (medium-to-high personal risk and above) and have a minimum of three injury crashes in five years.
3. The estimated DSI casualty equivalents per 100 million vehicle kilometres travelled (based on the latest five-year period) is ≥5 (medium personal risk and above) and have a collective risk of medium and a minimum of three injury crashes in five years.
4. The infrastructure risk rating(external link) (IRR) is high or medium-to-high and the estimated DSI casualty equivalents value (based on the latest five-year period) results in a medium collective risk or above. This is based on figure 1 below for urban (speed limit ≤70km/h) and figure 2 below for rural (speed limit ≥80km/h) speed environments respectively and have a minimum of three injury crashes in five years.

Note: For road sections with an annual average daily traffic of less than 1,500 vehicles per day, you may use 10 years of crash data instead of five to increase the size of the crash sample. Where 10 years of crash data are used, the minimum number of injury crashes increases from three to five.

Figure 1:  Urban (≤70km/h) corridor with estimated DSI casualty equivalents for collective risk thresholds. This is based on estimated DSI casualty equivalents per kilometre per year.

Figure 2: Rural (≥80km/h) corridor with estimated DSI casualty equivalents for collective risk thresholds. This is based on estimated DSI casualty equivalents per kilometre per year.

A high overall safety risk for intersections is where:

1. collective risk is ≥1.1, estimated using DSI casualty equivalents based on the latest five-year period.  This is equivalent to the high or medium-to-high collective risk definition in the High-risk intersections guide.
2. personal risk is ≥32, estimated using DSI casualty equivalents per 100 million vehicle kilometres travelled. This is based on the latest five year period,and meets the high personal risk definition in the High-risk intersections guide.

Any intersection, corridor or route that does not satisfy any of the high or medium safety risk definitions is classified as low safety risk.

A simplified method may be used by multiplying each injury crash by the corresponding DSi severity factor in table 1. 

Table 1: Generic corridor DSI severity indices

Use the Urban KiwiRAP(external link) method of classifying crashes at mid-blocks, and multiplying each injury crash by the corresponding DSI severity index (table 2). This index covers primary crash movement type, speed environment and midblock form.

Table 2: Corridor severity indices (source: Urban KiwiRAP(external link))

Use the method described in the High-risk intersections guide. Tabls 3 and 4 contain severity indices for different intersection forms and controls  for urban (speed limit ≤70km/h) and rural (speed limit ≥80km/h) speed environments respectively.

Table 3: Urban (≤ 70km/h) intersection severity indices (source: High-risk intersections guide)

Use the method described in Safer Journeys for motorcycling on New Zealand roads guide.